The present invention, in some embodiments thereof, relates to the generation of light and, more particularly, but not exclusively, to the generation of high harmonics of infrared light.
Lasers can be categorized by the ranges of wavelengths characterizing the electromagnetic spectrum in which they operate. These ranges include the infrared (IR) range, visible range and ultraviolet (UV) range. The greatest number of laser light sources is found in the IR and visible ranges of the spectrum while fewer options are available in the UV range.
The UV range is important because photons of these wavelengths contain enough energy to break chemical bonds and because short wavelengths can be focused more precisely than longer wavelengths. Additionally, the short wavelengths characterizing the UV range enable high resolution. UV light sources are used in applications such as spectroscopy, optical testing, medicine, machining and lithography.
Common UV sources include lasers that rely upon the harmonic conversion of light from sources in the visible or IR ranges. Harmonic generation provides an alternative to direct generation of ultraviolet light. In this approach light is produced in the visible or IR regions and then converted to shorter wavelengths via a nonlinear optical process. In these devices, the frequency of the output is typically doubled, and the governing process is termed second harmonic generation (SHG).
Intense linearly-polarized monochromatic IR laser pulses which are focused onto a gas of atoms can also lead to the emission of photons at higher harmonics, a phenomenon known as high harmonic generation (HHG). Harmonic generation spectra typically have a non-linear region in which the intensity of optical output decreases as a function of the harmonic order, followed by a “plateau” region in which the intensity of the harmonics remain approximately constant over many orders. The highest frequency of the harmonic generation spectrum (HGS) is referred to as the cut-off frequency of the spectrum, or the plateau's cut-off. When expressed in energy units, the plateau's cut-off is the maximum photon energy producible via HHG.
A semiclassical three-step model (also known as “recollision model”) explains the HHG phenomenon as follows. Under the influence of the intense laser field, the electron of an atom tunnels out of the modified Coulomb potential to the continuum, gains kinetic energy as a free particle in the field and may recombine with the parent ion to release the sum of its kinetic energy and the ionization potential as a high energy photon (to this end see, e.g., P. B. Corkum, Phys. Rev. Lett., 71:1994, 1993; Lewenstein et al., Phys. Rev. A, 49:2117, 1994; and Schafer et al., Phys. Rev. Lett., 70:1599, 1993).
Known in the art are techniques for achieving HGS by contaminating a strong IR field with a second or more higher-frequency, typically in the UV range [Pfeifer et al. Phys. Rev. Lett. 97:163901, 2006; Pfeifer et al., Optics Letters 31:975, 2006; Dudovich et al., Nature Physics 2:781, 2006; Eichman et al., Phys. Rev. A50:R2834, 1994; and Gaarde et al., Phys. Rev. A54:4236, 1996]. In these techniques, the frequency of the additional UV field is in the plateau of the HGS as would be generated by the IR field alone. These techniques were successfully employed for inducing stimulated emission, single-photon ionization or multi-photon ionization [Kitzler et al., Phys. Rev. Lett, 88:173904, 2002; Zeitoun et al., Nature 431:426, 2004, Papadogiannis et al., Phys. Rev. Lett. 90:133902, 2003]